With recent increase in the number of crimes such as car thefts, thefts from cars and the like, it has been widely popularized to install a burglar alarm device in a vehicle to sound an alarm by detecting abnormal conditions of the vehicle (e.g., the invasion of criminals on vehicles, the breakage of car windows or stealing by a wrecker) (see, e.g., Japanese Patent Application Publication No. 2005-215544).
A burglar, however, can disable a vehicle burglar alarm device by separating a main battery from the alarm device. For that reason, there has been disclosed a vehicle burglar alarm device capable of sounding an alarm by using an internal battery provided inside the alarm device, even when the main battery is separated from the alarm device.
FIG. 3 is a block diagram showing a conventional vehicle burglar alarm circuit provided in a vehicle burglar alarm device.
The conventional vehicle burglar alarm circuit includes a main battery 11, an internal battery 12, a DC/DC converter 13, a driving circuit 14 and a sounding body 15.
The main battery 11 includes a lead rechargeable battery loaded in a vehicle. In this conventional case, an output voltage V11 of the main battery 11 is a DC voltage of 12V. The internal battery 12 includes a Ni-MH battery, a lithium battery or the like. In this conventional case, an output voltage V12 of the internal battery 12 is a DC voltage ranging from 4.8 V to 6.0 V. Further, the DC/DC converter 13 employs the main battery 11 as an input power source when the main battery 11 is loaded, and employs the internal battery 12 as an input power source when the main battery 11 is separated. Further, there is provided a switch SW including a semiconductor device such as a field-effect transistor (FET), a transistor or the like for turning on and off a power supplied from the main battery 11 to the DC/DC converter 13.
The DC/DC converter 13 is of a flyback type and configured such that a boosting (step-up) ratio of an output voltage to an input voltage is variable. Further, the DC/DC converter 13 boosts the output voltage V11 and the output voltage V12 having different voltage values at different ratios to output a constant output voltage V13. In the conventional case, the output voltage V13 of the DC/DC converter 13 is a DC voltage of 40 V.
The driving circuit 14 includes four transistors (not shown). Further, the driving circuit 14 generates a high frequency output voltage V14 from the output voltage V13 of a DC voltage by turning on and off the four transistors. In this conventional case, the output voltage V14 from the driving circuit 14 is a high frequency voltage of 80 Vpp.
The sounding body 15 is a siren including a ceramic oscillator. When the output voltage V14 is applied to the sounding body 15, the sounding body 15 produces an alarm sound.
With the above configuration, the conventional vehicle burglar alarm circuit can produce an alarm sound having the same sound pressure level from the sounding body 15 regardless of the presence of the main battery 11.
The DC/DC converter 13 of the conventional vehicle burglar alarm circuit outputs the output voltage V13 having a predetermined value by varying a boosting ratio even though the main battery 11 and the internal battery 12 output different voltages. Accordingly, the DC/DC converter 13 requires a component for varying a boosting ratio, thereby complicating the circuit configuration.
Further, after the DC/DC converter 13 boosts the output voltage from the main battery 11 or the internal battery 12, the driving circuit 14 converts it into a high frequency voltage. Accordingly, it is necessary to install both the DC/DC converter 13 and the driving circuit 14, thereby increasing the number of parts and the cost.